Question

If the mass of cr and o are in the ratio 13:12 in cro3 then ratio of o that combines with the same mass of cr in cro3 and cro7 is

Answer 1

The equation for the reaction of oxygen and chromium is as follows: 

      4Cr(s)+3O₂(g)→2Cr₂O₃(s)

We can take a sample of the 2 where we have 13g of Cr and 12 g of O to have a mass ratio of 13:12, for the purpose of calculation.  

In the first reaction we will use a mass of oxygen of 12 g.

The reaction of oxygen and Chromium to from dichromate (CrO₇) is:

         2Cr + 7O₂ ---------------->  2CrO₇

Mole ratio is 2:7:2

In this case the same amount, 13 g of Cr will be used in this equation.

Calculate the moles of Cr (molar mass = 52)

moles = mass/molar mass
            = 13/52
            = 0.25 moles
mole ratio between Cr:O₇ is 2:7 , meaning the moles of oxygen will be ( 0.25 × 7/2 = 0.875) moles

Therefore the moles of oxygen are 0.875, calculate the mass using this mole value

mass = moles × molar mass   (molar mass of O₂ = 32)

        =  0.875 × 28 
    
        = 28 g 
The mass of oxygen used by same mass, 13 g of chromium to give Cr₂O₇ is 28g

The ratio of the oxygen in the 2 equations then is: 

12 g : 28 g    (simplify)

The ratio therefore is 3:7